Overvoltage protection is employed in a variety of electronic devices and applications. For example, in integrated circuit (IC) designs, there are typically circuit nodes which require protection from high voltage so as to reduce the likelihood of damage to certain circuit elements in a circuit to be protected. Such high voltage may be defined as a voltage greater than that which the IC fabrication process will allow to ensure reliability of the IC. This could happen, for example, at boundaries across multiple voltage domains, for electrostatic discharge (ESD) protection, etc. Such overvoltage protection is conventionally performed by shunting a transistor to certain protected circuit nodes, where the shunting transistor provides a current discharge path and limits the voltage at the circuit node being protected. The shunting transistor should be turned on during the overvoltage event so as to conduct current and limit the voltage at the node to be protected when the voltage exceeds a prescribed threshold level, but must be turned off during normal operation to avoid excessive power consumption in the circuit and/or avoid limiting/saturating the useful signal swing.
One problem with conventional overvoltage protection circuitry is that the shunting transistor typically experiences large IC process variations so that the threshold voltage at which the transistor turns on, e.g., the lowest voltage that can turn on the transistor, may change over a wide range. This makes it difficult to protect sensitive circuit nodes, since if the threshold voltage is too high, the transistor will not turn on in time to protect the nodes efficiently. Alternatively, if the threshold voltage becomes too low, the circuit will cut off the signal voltage during normal operation by shunting to ground the circuit node to be protected.
Accordingly, there exists a need for an improved overvoltage protection circuit which does not suffer from one or more of the problems exhibited by conventional overvoltage protection circuits.